The disclosure of Japanese Patent Application No. 2001-097854 filed on Mar. 30, 2001, including the specification, drawings and abstract, is incorporated herein by reference in its entirety.
1. Field of the Invention
The invention relates to a shift control apparatus of an automatic transmission installed in a motor vehicle and, more particularly, to a speed shift control apparatus for a downshift.
2. Description of the Related Art
In the conventional art, when a downshift is determined, the downshift is accomplished by reducing the oil pressure on a releasing-side friction engagement element in the transmission to an initial pressure calculated from the degree of throttle opening, the input torque or the target shift time, thereby releasing the releasing-side friction engagement element.
Downshift commands can be roughly divided into a downshift command prompted by gradual depression of the accelerator pedal as in climbing a hill, and a downshift command prompted by rapid depression of the accelerator pedal, e.g., for overtaking another vehicle. The former is a downshift that is suddenly started when the driver does not intend a downshift at all. In this case, a shift operation with a reduced shock is desirable instead of a quick shift operation responsive to the accelerator operation. In the latter downshift, quick response of the shift operation to the accelerator operation is regarded as more important than reduced shift shock.
However, if response-critical control is performed for a downshift, a rather hard shift shock is inevitable, and shock will also occur during a downshift on a sloped road grade. Conversely, if shock-critical control is performed so as to avoid a shift shock, a quick response cannot be provided at the time of acceleration in passing and the like. Thus, there is the problem of providing a shift of a type in accordance with the driver""s intention.
The downshift point could be changed in accordance with the speed of throttle pedal depression at the time of output of a downshift command. Likewise, it is conceivable the oil pressure for a hydraulic servo that operates a friction engagement element might be changed in accordance with the speed of accelerator depression. However, the speed of throttle pedal depression cannot be accurately detected at a pedal position near the maximum extent of depression of the accelerator pedal (full down), i.e., a position near the kick-down point, because at such a position, the remaining available stroke of the accelerator pedal is small. Since the speed of depression of the accelerator pedal near the kick-down point cannot be accurately detected, the prior art always performs the shift operation in a shift shock-critical fashion and therefore suffers from the drawback of insufficient responsiveness in passing situations, etc.
Accordingly, it is an object of the invention to provide speed shift control of an automatic transmission which properly determines the type of downshift and thereby provides a shift feel in accordance with a driver""s intention.
The present invention provides a unique downshift control for a conventional automatic transmission which includes an input shaft receiving power from an engine output shaft, an output shaft operatively connected to the drive wheels, a plurality of frictional engagement elements selectively engaged to change the torque transfer path between the input shaft and the output shaft; and hydraulic servos that engage and disengage the friction engagement elements to effect the shift change. The control apparatus, of the apparatus of the present invention, which controls a downshift in such a transmission, includes a controller which receives signals from sensors for detection of various vehicle running conditions and outputs oil pressure control signals in accordance with the sensor input signals. The controller includes a control means for controlling the oil pressure to the hydraulic servos operating friction engagement elements which participate in the downshift to a predetermined speed stage, responsive to a downshift command. The controller further includes downshift type determining means for determining whether the downshift should be a type oriented toward minimum shock or a type oriented toward responsiveness, based on a detected degree of throttle opening and a detected speed of accelerator pedal depression. The control means controls the oil pressures to the hydraulic servos for the friction engagement elements based on the downshift type determined by the downshift type determining means.
The control means may include engaging side control means for controlling the oil pressure to the hydraulic servo for a friction engagement that is on the engaging side in the downshift and releasing side control means for controlling the oil pressure to the hydraulic servo for a friction engagement element that is on a releasing-side in the downshift. In this case, the releasing-side control means controls the oil pressure to the hydraulic servo for the friction engagement element on the releasing-side, based on the downshift type determined by the downshift type determining means. The downshift type determining means may take into account whether the downshift command has resulted from automatic operation of the transmission or a manual downshift by the driver.